TV signals currently used widely can be classified into a standard TV signal having an exact format to the NTSC (National Television System Committee), and a non-standard TV signal, such as reproduction signal of a video cassette tape recorder. The standard TV signal has a frequency relation between horizontal synchronization signal and sub-carrier signal exactly meeting the following equation (I), whereas the non-standard TV signal may not. EQU f.sub.sc =455f.sub.H /2 (I)
Wherein, the f.sub.sc is the frequency of a color sub-carrier signal of about 3.58 MHz and the f.sub.H is the frequency of a horizontal synchronization signal of 1570 Hz.
Therefore, a non-standard TV signal, which currently is used with an analog type TV system, does not exhibit a significant picture quality degradation. However, an IDTV system, with digital signal processing, can exhibit significant picture quality degradation depending on the clocks used, i.e., BL clock dedicated to a color burst signal, and LL clock dedicated to a horizontal synchronization signal.
The digital TV system has, for main signal processing parts, a luminance/color separator, color demodulator, and a line doubler. In case the BL clock is used for signal processing, a comparatively good result can be obtained from the luminance/color separator and the color demodulator, but a great picture quality degradation of screen flicker occurs for the non-standard TV signal processing of the line doubler. When the LL clock is used, a good result without screen flicker can be obtained from the processing of the line doubler. A great picture quality degradation in not being able to obtain a color signal, can occur for the non-standard TV signal processing of the luminance/color separator and the color demodulator.
Therefore, in an IDTV system, signal processing paths are changed according to the result of detection of standard and non-standard TV signals, BL clock is used for luminance/color separation and color demodulation, and, after the color demodulation, a sample rate of signals processed using LL clock as well as BL clock, i.e., luminance/color separated and color demodulated signals is converted suitable to LL clock and applied to a circuit part, using the LL clock.
The foregoing conventional art is explained hereinafter, referring to FIGS. 1-4(c).
As shown in FIG. 1, a conventional digital signal processing circuit for processing digital signal through detecting standard/non-standard TV signal includes a luminance/color separator 1 for separating luminance signal Y and color signal C from an analog composite video signal input. An analog color demodulator 2 for demodulating color difference signals U and V from the color signal C is connected to the luminance/color separator 1. Analog/digital converter 3 converts the luminance signal Y connected to the luminance/color separator 1 into a digital signal. The color difference signals U and V which are demodulated at the analog color demodulator 2 are converted into digital signals by analog/digital converters 4, 5. A digital signal processor 6 processes the signals received from the analog/digital converters 3, 4 and 5. Digital/analog converters 7, 8 and 9 convert the signals received from the digital signal processor 6 into an analog signal. A standard/non-standard detector 10 detects the received analog composite video signal of a standard TV signal and a non-standard TV signal. A PLL part 11 for LL, and a PLL 12 for BL, generates the LL clock and BL clock respectively. A change over switch 13 selects signals received from the PLL part 11 for LL, and the PLL part 12 for BL, and applies the selected signal to the analog/digital converters 3, 4 and 5, the digital signal processing part 6, and the digital/analog convertors 7, 8 and 9, as clock pulses on signals in response to the signal received from the standard/non-standard detector 10.
Operation of the foregoing conventional digital signal processing circuit through detecting standard/non-standard TV signal is explained hereinafter.
The composite video signal input is separated into luminance signal Y and color signal C at the luminance/color separator 1, and the separated color signal C is demodulated into color difference signals U and V at the analog color demodulator 2.
The separated luminance signal Y and the color difference signals U and V are converted into digital signals at the analog/digital converters 3, 4 and 5 respectively, processed by the digital signal processor part 6, and converted into analog signals at the digital/analog converter 7, 8, and 9, respectively.
The standard/non-standard detector 10 detects the received analog composite video signal of a standard TV signal and a non-standard TV signal and applies a control signal to the change over switch 13 according to the result of detection. The PLL part 11 for LL generates LL clock pulses dedicated to the horizontal synchronization signal, and the PLL part 12 for BL generates the BL clock pulses dedicated to color burst signals and applies the BL clock output or pulses to the change over switch 13. The change over switch 13 changes over in response to the signal received from the standard/non-standard detector 10 to select LL clock of the PLL part 11 for LL, or BL clock of the PLL part 12, for BL to apply to the analog/digital converters 3, 4 and 5, the digital signal processing part 6 and the digital/analog converters 7, 8 and 9, as clock, i.e., in case it is a standard TV signal, the BL clock is applied, and in case it is non- standard TV signal, the LL clock is applied.
However, the foregoing conventional art has problems in that the standard/non-standard detector 10 for detecting the standard/non-standard TV signal is difficult to manufacture because it is complicated, hard to integrate because it is processed in analog format up to the color demodulation, and is expensive because of the addition of the analog function and the conversion of the component signal into a digital signal.
As shown in FIG. 2, a conventional analog sample rate conversion device includes an analog/digital converter 21 for converting an analog composite video signal input into a digital signal according to the BL clock. A luminance/color separator and color demodulator 22 for luminance/color separating and color demodulating of the signal received from the analog/digital converter 21 according to BL clock is included. A digital/analog converter 23 is for converting the signal received from the luminance/color separator and color demodulator 22 into an analog signal according to the BL clock. A low pass filter 24 for low pass filtering of the signal received from the digital/analog converter 23, and an analog/digital converter 25 for converting the signal received from the low pass filter 24 a into digital signal according to LL clock is included. A memory application part 26 for carrying out the signal processing using memory, such as line doubling and ARC (Aspect Ratio Conversion) of the signal received from the analog/digital converter 25 according to LL clock, and a digital/analog converter 27 for converting the signal received from the memory application part 26 into analog signal according to LL clock, and a low pass filter 28 for low pass filtering of the signal received from the digital/analog converter 27 are also included.
Operation of the foregoing conventional analog sample rate conversion device is explained hereinafter.
The analog composite video signal is applied, under a condition that BL clock and LL clock are being applied thereto, is converted into a digital signal at the analog/digital converter 21 according to BL clock, and separated into luminance signal and color signal, as well as the separated color signal being demodulated into color difference signal at the luminance/color separator and color demodulator 22 according to the BL clock. The luminance signal and the color difference signal are converted into analog signals respectively at the digital/analog converter 23 according to the LL clock.
The analog luminance signal and color difference signal received from the digital/analog converter 23, are low pass filtered at the low pass filter 24, converted into digital signals at the analog/digital converter 25 according to the LL clock, line doubled and ARC processed at the memory application part 26 according to the LL clock, converted into analog signals at the digital/analog converter 27 according to the LL clock, and low pass filter at the low pass filter 28.
Although, the foregoing conventional sample rate conversion device does not develop any problems due to the difference of the BL clock and the LL clock for a non-standard TV signal because the digital luminance signal and color difference signal luminance/color separated and color demodulated according to BL clock are signal processed at the memory application part 26 after converted into analog signals, and converted again into digital signals according to LL clock, it has problems in that it inevitably develops picture quality degradation due to mixing with many noise signals in the process of converting the digital luminance signal and the color difference signal into analog signals, and then converting into digital signals again after the low pass filtering. It is expensive because many expensive analog/digital converters and digital/analog converters are used in the system.
As shown in FIG. 3, a conventional digital sample rate conversion device includes an analog/digital converter 31 for converting an analog composite video signal input into a digital signal according to the BL clock; a luminance/color separator and color demodulator 32 for luminance/color separating and color demodulating of the signal received from the analog/digital converter 31 according to BL clock; a sample rate converter 33 for receiving the signals transmitted from the luminance/color separator and color demodulator 32 according to BL clock; and transmitting the signals synchronized to LL clock; a memory application part 34 for carrying out signal processing using memory, such as line doubling and ARC (Aspect Ratio Conversion) of the signal received from the sample rate converter 33 according to LL clock; a digital/analog converter 35 for converting the signal received from the memory application part 34 into an analog signal according to LL clock; and a low pass filter 36 for low pass filtering of the signal received from the digital/analog converter 35.
Operation of the foregoing conventional digital sample rate conversion device is explained hereinafter.
An analog composite video signal applied under the condition that the BL clock and LL clock are being applied, is converted into a digital signal at the analog/digital converter 31 according to BL clock, and separated into a luminance signal and color signal. The separated color signal is demodulated into a color difference signal at the luminance/color separator and a color demodulator 32 according to BL clock.
The luminance signal and color signal received from the luminance/color separator and color demodulator 32, are applied to the sample rate converter 33, transmitted therefrom in synchronization with LL clock, line doubled and ARC processed at the memory application part 34 according to the LL clock, converted into analog signals at the digital/analog converter 35, and low pass filtered at the low pass filter 36.
The Operational principle of the digital sample rate converter 33 based on interpolation method for the conventional digital sample rate conversion device is explained with references to FIGS. 4(a)-4(c).
The sampling component shown in FIGS. 4(a) and 4(b) in solid lines is the signal component applied to the sample rate converter 33 from the luminance/color separator and color demodulator 32, i.e., the signal component digitally processed by the BL clock.
The sampling component shown in FIG. 4(b) in dotted lines is the signal component transmitted from the sample rate converter 33 according to LL clock.
As shown in FIG. 4(c) the sample rate can be converted by an interpolation filter (not shown) having a variable filter coefficient obtained from the phase information on the BL clock and LL clock through the process of converting the signal component synchronized with the BL clock to synchronize with the LL clock at the sample rate converter 33.
However, the conventional digital sample rate converter has a problem in that the calculation of the exact variable filter coefficient is not possible because the required exactness of the phase comparison of the BL clock and the LL clock for calculating the variable filter coefficient of the interpolation filter should be carried out at a frequency more than 1000 times higher than the frequency of LL clock. For example, in case the frequency of LL clock is 910 f.sub.H (.about.14.37 MHz), it should be carried out at a very high frequency of about 14.3 GHz for calculating an exact variable filter coefficient.